mgr: generate crash dump for python exceptions

mgr: generate crash dump for python exceptions

Extend handle_pyerror() to generate a crash dump.  Pass some additional
context through from the callers (including the ability to not generate
a crash dump in the CLI handler case).

Extra crash dump fields look like so:

    "backtrace": [
        "  File \"/home/sage/src/ceph/src/pybind/mgr/balancer/module.py\", line 652, in serve\n    self.ifail()",
        "  File \"/home/sage/src/ceph/src/pybind/mgr/balancer/module.py\", line 648, in ifail\n    raise RuntimeError('test')",
    ],
    "mgr_module": "balancer",
    "mgr_module_caller": "PyModuleRunner::serve",
    "mgr_python_exception": "RuntimeError",

Notably, the backtrace deliberately excludes the 'value' of the exception,
as that may leak identifying information about the system.  Instead, we
only include the exception *type* and the portion of the traceback that
identifies the call path (where in the code we crashed).

Also note: a side-effect of this change is that module exceptions will
trigger cluster health warnings about daemon crashes.

Signed-off-by: Sage Weil <sage@newdream.net>